   edges only.  Example: {:a #{:b}} => {:a #{:b}, :b #{}}"
   [g]
   (let [have-incoming (apply union (vals g))]
-    (reduce #(if (get % %2) % (assoc % %2 #{})) g have-incoming)))
+    (reduce #(if (% %2) % (assoc % %2 #{})) g have-incoming)))
 
 (defn topo-sort
   "Proposes a topological sort for directed graph g using Kahn's

     (topo-sort (normalize g) [] (no-incoming g)))
  ([g l s]
     (if (empty? s)
       (when (every? empty? (vals g)) l)
       (let [[n s'] (take-1 s)
             m (g n)
             g' (reduce #(update-in % [n] without %2) g m)]
         (recur g' (conj l n) (union s' (intersection (no-incoming g') m)))))))


(ns tailrecursion.boot.kahnsort
  (:require
   [clojure.set :refer [difference union intersection]]))

(defn choose
  "Returns the pair [element, s'] where s' is set s with element removed."
  [s] {:pre [(not (empty? s))]}
  (let [item (first s)]
    [item (disj s item)]))

(defn no-incoming
  "Returns the set of nodes in graph g for which there are no incoming
